In a gas turbine engine, a portion of the incoming airflow from the compressor may be diverted to cool various turbine components. The diverted air, however, may consume a large portion of the total airflow through the compressor, as much as about twenty percent. Improved management and control of these parasitic air flows therefore may increase the overall performance and efficiency of the gas turbine engine.
For example, air may be extracted from the compressor for the use as a cooling flow for various turbine components. The extracted air thus bypasses the combustion system. Ejectors are often useful for this purpose. Bleed air may be extracted from, for example, a thirteen stage of the compressor to cool a second stage nozzle of the turbine. Bleed air also may be extracted from another stage, for example, a ninth stage at a lower pressure and temperature than is extracted from the thirteenth stage for supplying cooling air to a third stage nozzle of the turbine. The extraction ports, however, often provide the cooling airflow at too high a pressure and/or temperature. The flow thus may be throttled to the desired pressure and/or temperature. This throttling, however, may result in a net loss of energy. By employing an ejector, a low pressure/temperature airflow may be mixed with a high pressure/temperature airflow to provide an airflow at an intermediate pressure and temperature so as to manage the pressure and temperature required to cool the turbine stages. The ejector thus makes use of the low pressure and temperature airflow that otherwise may be dissipated as waste energy.
An ejector generally does not have any moving parts. Rather, the ejector is sized to provide optimally the required airflow at ISO conditions (typically about 59 degrees Fahrenheit (about 15 degrees Celsius)). Daily temperature variations, however, will have an impact on the operational characteristics of the ejector. In other words, the ejector may behave differently on different days and at different times during each day. On hot days, the ejector may deliver more air than required and thus may overflow. Such an overflow may not impact the lifetime of the ejector, but the performance benefits may suffer. The optimum operating conditions for an ejector generally occur on hot days (above about 70 degrees Fahrenheit (about 21.1 degrees Celsius) and at part loads (e.g., below about a fifty percent load on the turbine). Conversely on cold days, a bypass line parallel to the ejector may be required to provide an additional cooling flow that the ejector cannot supply alone.
There is thus a desire for an ejector system that can accommodate daily variation in ambient conditions. Such a system preferably may increase the output and efficiency of the gas turbine engine as a whole.